The present invention relates generally to an apparatus and a method for focusing an electron beam generated from a cold cathode electron emitter, and more particularly to an apparatus and a method with an ion shield for focusing a high-current-density electron beam generated from a planar cold cathode electron emitter.
Field emission has been extensively used in characterization of material surface structure and electronic properties. Apart from the surface physics, field emission at present, has gained a different importance in technology. Field emitters can be used as cathodes for electron emission applications because of the superior emission properties.
At present, thermionic cathodes are employed exclusively in applications that require high-density electron beams. Replacement of these thermionic cathodes by high-density cold cathodes is predicted to allow performance unachievable by these thermionic emitters. For a planar, high current density cold electron source such as a field emitter array (FEA) or a wide bandgap material, though high current density of electron beam can be generated due to its inherently compact nature, electron beam control is a challenge before practical applications for high power device. As the cold emitters are generally non-convergent, that is, as the surface of the emitter is planar and the resulting beam has a natural tendency to defocus due to the large space charge forces created by the high current density, the difficulty in controlling the electron beam is further exacerbated.
Beam emittance is another issue. Due to the nature of emission process, cold emitters generally produce beams with perpendicular velocity spreads several times that of the beams produced by space charge limited thermionic emitters. This can result in beam interception on the focusing elements or poor beam confinement once the beam has been injected into a confining magnetic field. Therefore, to design an apparatus which focuses an electron beam created by a high-density planar cold cathode emitter, issues of beam emittance must be addressed during the design process.
An apparatus and a method of focusing a high-current-density electron beam emitted from a cold cathode electron emitter are provided to overcome the problems occurring in the prior art. A series of shaped electrostatic lenses are located in front an emission surface of the cold cathode electron emitter. By applying different focusing voltages to the electrostatic lenses simultaneously, the high-density-current electron beam is well focused with a laminar profile and well-confined in the magnetic field in the travel wave tube. The magnitude of focusing voltage applied to each of the electrostatic lenses is limited to a range that will well focus the electron beam and well confine it within the magnetic field.
In the above apparatus, the cold cathode electron emitter comprises a non-convergent emission surface, from which the high-current-density electron beam is emitted. In one embodiment of the invention, four shaped electrostatic lenses are used. The electrostatic lenses are electrically isolated from each using an isolation ceramic. The cold cathode electron emitter further comprises a weld flange holding an anode in front of the series of electrostatic lenses. Again, the isolation ceramic is used to electrically isolate the anode from the electrostatic lenses. Physically, between every two neighboring electrostatic lenses, and between the emission surface and the electrostatic lenses, there is located an isolation ceramic. Further, an ion shield is inserted in front of the emission surface, which applies a positive potential between the high-voltage emission surface and a grounded body of the device to which the electron gun is attached. The magnitude of the positive potential is sufficiently large to screen the ion bombardment.
In one embodiment of the invention, the above apparatus and method provides a well-focused laminar electron beam with a current density between 0 A/cm2 to 20 A/cm2.